This invention relates generally to cellular communications systems. More particularly, it relates to systems and methods for dynamically allocating centralized capacity resources to remote cells in a CDMA cellular network.
As cellular communications rapidly spread into every walk of modern life, there is a growing demand for ever greater service at ever lower cost.
Conventional cellular networks employ an architecture which divides a geographical area into coverage areas called cells, and a base-station is placed at the center of each cell to serve the cellular traffic. The base-station is equipped with transmitters and receivers that provide the RF radio coverage, while a fixed number of radio channels in the base-station determines the traffic handling capacity. Since each cell must be provided with an adequate number of radio channels in order to meet the peak traffic demand with a specified grade-of-service, the cost for providing such peak traffic capacity and the associated operational expenses must be paid at the outset, though the peak traffic capacity may not be fully utilized most of the time. The situation is further compounded by the non-uniform and dynamic nature of the traffic capacity demand within the cellular network, resulting in capacity shortages in some of the cells while capacity excesses others experience. Moreover, as the demand for cellular service increases within a particular area, the network must be re-engineered and more base-stations must be installed to meet the demand, which can be costly and time consuming. All in all, the dynamic nature of traffic capacity demand makes it difficult for the current cellular networks to operate efficiently and to optimize both cost and grade-of-service.
U.S. co-pending patent applications, xe2x80x9cAdaptive Capacity Management in a Centralized Base-station Architecturexe2x80x9d of Adam Schwartz Ser. No. 09/560,656 filed on Apr. 27, 2000, and xe2x80x9cA Cellular Communications System With Centralized Capacity Resources Using DWDM Fiber Optic Backbonexe2x80x9d of Woon Wong and Adam Schwartz 09/561,372 filed on Apr. 28, 2000, provide a novel cellular network architecture that de-couples the traffic capacity and the RF coverage in a cellular network by placing base-stations at a centralized location, in contrast to one base-station per cell structures in prior art networks. The RF coverage in each remote cell is independently provided by one or more RF antennae placed inside the cell. Such a centralized base-station architecture enables the cellular network to dynamically allocate traffic channels to remote cells based upon traffic demand and grade-of-service requirement in each cell, thereby enhancing overall capacity in the network. More specifically, 09/560,656 provides an adaptive capacity management method for cellular communications systems in which radio resources utilize non-interfering channels, such as frequency bands in Frequency Division Multiple Access (FDMA), or time-slot assignments in Time Division Multiple Access (TDMA). 09/561,372 provides a cellular network in which optical fibers and Dense Wavelength Division Multiplexing (DWDM) are advantageously employed to distribute multiple traffic channel groups from the centralized base-stations to different remote cells. The present invention addresses cellular communications systems in which radio resources employ wide-band Code Division Multiple Access (CDMA) channels.
In a CDMA cellular system, the basic unit of radio resource is a set of orthogonal digital codes whose frequency spectrum is spread over a given band of frequency by a pseudo-noise (PN) digital sequence (a spreading code). More than one PN sequence are used to spread the digital codes in a given frequency band. Each digital code spread by a PN sequence is referred to as a CDMA channel, hereinafter. A traffic channel group consisting of one or more CDMA channels characterized by the same PN code occupying the same frequency band is referred to as a CDMA signal, hereinafter. In CDMA technology, PN codes are used for a variety of purposes. The use of PN codes in this invention refers solely to the spreading code used to spread downlink CDMA signals for the purpose of distinguishing downlink CDMA signals of the same frequency band from one another. While CDMA channels within each CDMA signal are orthogonal to each other, CDMA channels belonging to different CDMA signals are not orthogonal to each other. Therefore, when CDMA channels belonging to different PN sequences (i.e., different CDMA signals) are used simultaneously in a cell, cross-interference will occur amongst CDMA channels, which degrades the signal-to-noise ratio of channel reception and leads to undesirable soft-handoff.
Hence, while there is no inherent limit to the number of non-interfering FDMA or TDMA channels that can be shuffled to a given cell, so long as the frequency spectrum and other physical constraints permit, there is an upper limit to the number of CDMA channels sharing a common frequency band that can be allocated to a cell. That is to say that in the current state of CDMA cellular communications, the number of users that can be supported in a cell is limited by the cross-interference amongst CDMA channels, rather than by the amount of traffic channel resources that can be devoted to it.
Sectorization has been implemented in the art to mitigate the cross-interference amongst CDMA channels as described above. That is, multiple directional antennae are used to divide a cell into multiple sectors with mutually exclusive radio coverage areas, such that different CDMA signals allocated to the cell are assigned to different antennae. However, the allotment of traffic channel resources to each sector in the prior art cellular networks has been on a fixed basis, with no provision for dynamic assignment of traffic channels based upon traffic demand and grade-of-service requirement.
What is needed in the art are therefore cellular communications systems in which traffic capacity resources are dynamically managed and optimally utilized.
Accordingly it is a principal object of the present invention to provide a cellular network architecture in which traffic channel resources are centralized and dynamically allocated to remote cells according to the demand. It is another object of the present invention to provide a method for maximizing capacity resources by dynamically sectorizing cells in a CDMA cellular network. It is a further object of the present invention to provide a cellular communications network in which centralized traffic channel resources are distributed to remote cells by use of Wavelength Division Multiplexing (WDM) on optical fibers and remote cells are dynamically sectorized according to traffic demand and grade-of-service requirement.
The primary advantage of the present invention is that it enables a CDMA cellular network to dynamically manage and optimally utilize its capacity resources without having to change its hardware design, in contrast to the static and passive nature of the prior art cellular networks. The present invention provides a cost-effective buildout strategy for cellular network operators. Another advantage of the present invention is that as the demand for cellular service increases in a particular area, more capacity can be easily implemented without disrupting the overall operation of the entire network. A further advantage of the present invention is that the use of optical fibers and WDM provides a simple, efficient, and economical way to transmit traffic channel resources between centralized base-stations and remote cells.
These and other objects and advantages will become apparent from the following description and accompanying drawings.
The present invention provides a cellular network, including a centralized base-station site containing a plurality of base-station units, one or more remote cells, each equipped with S directional antennae, a cellular distribution means for transmitting traffic channel resources between the centralized base-station site and the remote cells, and a management system for supervising traffic channel allocation within the network.
In the cellular network of the present invention, the base-station site is placed at a location that may or may not physically overlap with any of the cell sites. The key feature is that base-station units are clustered together, as opposed to one base-station per cell structures in prior art cellular networks. Each base-station unit handles one or more CDMA signals each containing n CDMA channels at a given frequency band. The S antennae in each remote cell, typically but not necessarily of directional radiation patterns, covers approximately equal and non-overlapping sections of the cell. The cellular-distribution means can be one or more optical fibers along with corresponding units for making the conversion between cellular signals and optical signals, and for multiplexing/de-multiplexing optical signals to the optical fibers. The management system is in communication with the centralized base-station site and capable of measuring offered traffic in each cell, defined as the time-averaged number of simultaneous on-going calls taking place in that cell.
In the initial buildout of the cellular network when traffic is relatively light, one CDMA signal containing n CDMA channels is assigned to all S directional antennae in each remote cell. The management system monitors the offered traffic in each of the remote cells within the entire network and inputs the measured offered traffic to an optimization algorithm. As the traffic grows, the offered traffic m in a given cell may approach the maximum number of simultaneous call which the CDMA signal can support, which generally is less than n due to interference arising from, among other things, non-perfect isolation between different cells and cell sectors. The management system executes the optimization algorithm to determine the number of CDMA signals to be assigned to each cell, and through allocation of different CDMA signals to different antennae with a cell, sectorizes those cells that are assigned with more than one CDMA signal. The sectorization is physically accomplished through the use of antennae that provides mutually exclusive coverage areas in each cell, such that each antenna is assigned no more than one CDMA signal and each sector is served by one CDMA signal. For instance, if three CDMA signals are allocated to a cell where there are three directional antennae with non-overlapping radio coverage areas, each directional antenna is assigned one CDMA signal, and the cell is divided into three sectors. This assignment of distinct CDMA signals to different directional antennae provides more traffic channel capacity to the cell, while mitigating the cross-interference among different CDMA signals.
Various optimization algorithms can be implemented in the management system described above. One exemplary optimization algorithm works essentially as follows. The algorithm assigns to each remote cell a fraction of the total number of CDMA signals available at the base-station site, where the fraction is taken to be approximately equal to the ratio of the offered traffic in a remote cell to the total offered traffic in all remote cells, subject to the constraints that (a) the number of CDMA signals assigned to each remote cell does not exceed S, the maximum number of sectors into which a remote cell can be sectorized, and (b) each remote cell is assigned at least one CDMA signal.
It should be noted that many other optimization algorithms with different optimization constraints and performance metric can also be implemented for the purpose of the present invention. A skilled artisan can devise a suitable optimization algorithm for a given application.
It should be pointed out that sectorization has been employed in cellular communications networks, in both FDMA and TDMA systems primarily for interference reduction and in CDMA systems for capacity enhancement. Such systems commonly use multiple directional antennae to divide a cell into multiple sectors. In contrast to the prior art sectorization mechanism where the allotment of traffic channel groups to different sectors in a cell is on a fixed basis, the present invention makes all traffic channel resources in a cellular network available to all cells and can dynamically sectorize, and further de-sectorize, a cell according to the traffic demand, thus enabling the network to maximize its capacity and operate more efficiently.
The novel features of this invention, as well as the invention itself, will be best understood from the following drawings and detailed description.